The invention relates to a panel-form loudspeaker utilizing a sound radiator panel that can generate beneficial vibrational normal modes for radiating sound with desired pressure level over a specific frequency range.
Conventional loudspeakers utilizing a cone-type membrane as a sound radiator have been in widespread use. The cone shape radiator which is mechanically driven at its smaller end and in a pistonic manner by a moving coil of electromagnetic means can radiate sound waves from the front and rear of the radiator. In general, an enclosure is necessary to prevent low-frequency waves from the rear of the loudspeaker, which are out of phase with those from the front, from diffracting around to the front and interfering destructively with the waves from the front. The existence of such enclosure makes the loudspeaker possess some disadvantages such as cumbersome, weighty, having dead corner for sound radiation, etc. The shortcomings of conventional loudspeakers have led to the intensive development of panel-form loudspeakers in recent years and many proposals have thus resulted. For instance, Watters used the concept of coincidence frequency, where the speed of sound in panels subject to bending wave action matches the speed of sound in air, to design a light stiff strip element of composite structure that can sustain bending waves and produce a highly directional sound output over a specified frequency range. Heron proposed a panel-form loudspeaker which had a resonant multi-mode radiator panel. The radiator panel which was a skinned composite with a honeycomb core was excited at frequencies above the fundamental and coincidence frequencies of the panel to provide, hopefully, high radiation efficiency through multi-modal motions within the panel. The design of such radiator panel, however, makes it so stiff that it requires a very large and cumbersome moving-coil driver to drive the panel and its overall efficiency from the viewpoint of electrical input is even less than conventional loudspeakers. Furthermore, the operating frequency range of the radiator panel is not wide enough for general purposes and thus only suitable for public address applications. Azima et al proposed a distributed mode method for the design of a panel-form acoustic device which consisted of a panel radiator capable of sustaining bending waves associated with resonant modes in the panel radiator and used transducers to excite the resonant modes of the panel radiator. Their proposed distributed mode method includes analysis of distribution of flexural resonant modes and identification of dead/combined dead-spots of the panel radiator. The transducers are mounted at some particular points on the radiating panel which, hopefully, will not be coincident with the dead/combined dead-spots. Such design, however, is too idealistic to be practical, especially for the design of laminated composite panel radiators. Since for a panel under vibration, there may be several thousand resonant modes with frequencies in the range from 50 to 20 kHz. It thus becomes extremely infeasible or even impossible to identify all dead/combined dead-spots of the panel. In face of this difficulty, they simplified the design process by using only lower orders of resonant modes in the design of panel radiator. The adoption of such simplification in the design of the panel radiator has thus caused the sacrifice of the performance of the loudspeaker. Since only finite number of dead/combined dead-spots on the radiating panel are identified, it is inevitable that the points at which the transducers are mounted will coincide with some of the dead/combined dead-spots of higher resonant modes. It then becomes obvious that the transducers mounted at the dead/combined dead-spots of certain resonant modes will be unable to excite those modes and the intensity of sound radiated from the panel vibrating at the corresponding frequencies will become too low to be acceptable. Their approach in determining the locations of the transducers also creates another shortcoming of the loudspeaker. Due to the existence of over six thousand resonant modes, the transducers will inevitably over-excite certain resonant modes and thus generate undesirable sound intensities or overshoots at the corresponding frequencies. Furthermore, the other major defect existing in their proposal is the interference of sound waves radiated from different regions on the panel radiator. On a vibrating panel, the sound waves radiated from the convex and concave regions on the panel surface are out-of-phase. The sound waves of opposite phase will generate interference among them and thus lower the sound pressure level. In particular, for a panel vibrating with resonant modes in lower frequency range, the interferences among the sound waves of opposite phase may be so severe that they will significantly lower the sound intensities at the corresponding frequencies. The problem of sound level reduction caused by the interference of sound waves of opposite phase, however, was not observed and tackled by the previous proposers. In view of the above disadvantages, it is apparent that the method proposed by Azima et al can only find limited applications on the fabrication of low efficient acoustic devices. As for the design of loudspeakers of high fidelity, their method is still far from reach.
It is, therefore, a principal object of the present invention to provide a panel radiator for a loudspeaker which can produce a desired sound pressure level spectrum over a predetermined frequency range. The panel radiator for a loudspeaker includes a thin laminated composite radiating plate with stiffened peripheral edge and a preselected number of transducers mounted on the laminated composite plate at specific locations in a predetermined feasible region. The laminated composite radiating plate, which consists of a preselected number of orthotropic laminae with predetermined fiber angles with respect to the laminate axes, is capable of radiating sound waves through flexural vibration of the plate when excited by the transducers. The area of the laminated composite radiating plate is divided into feasible and infeasible regions. A laminated composite plate with transducers mounted in the infeasible region radiates too low sound pressure level to be practical. On the contrary, sound pressure level above 80 dB can normally be achieved over a specific frequency range if the plate is excited by transducers mounted in the feasible region. The area of the feasible region is determined using the method revealed in the present invention. The peripheral edge of the laminated composite radiating plate is stiffened by strips of predetermined rigidities. Sound quality and radiation efficiency of the panel radiator over a desired acoustic frequency range are dependent on values of particular parameters of the radiator, including lamination arrangement of the laminated composite radiating plate, specific moduli of the composite laminae used for fabricating the radiating panel, rigidities of the edge strips, and locations of the transducers mounted in the feasible region on the laminated composite radiating plate. Proper selection of the values of the parameters can produce required achievable sound pressure level spectrum of the panel radiator for operation of the loudspeaker over a desired operative acoustic frequency range.
Another object of the invention is to provide a method for designing a laminated composite panel radiator which includes a laminated composite plate stiffened around its peripheral edge with strips of suitable rigidities and a number of transducers mounted on the surface of the laminated composite plate at predetermined locations in the feasible region on the radiating plate. Optimal values of particular parameters of the laminated composite panel radiator, including lamination arrangement of the laminated composite plate and specific moduli of the constituted composite lamina, the area of the feasible region on the laminated composite plate, rigidities of the edge strips, and locations of the transducers in the feasible region are selected in the design process to achieve the required sound pressure level spectrum of the panel radiator for operation of the loudspeaker over a desired acoustic frequency range.
The present invention may best be understood through the following descriptions with reference to the accompanying drawings, in which: